This invention concerns a susceptor for supporting a silicon wafer and, more specifically, it relates to a susceptor for supporting a silicon wafer during in-line coating or deposition on the silicon wafer by a chemical vapor deposition (CVD) process.
Generally, in the case of applying an in-line CVD coating on a silicon wafer, the silicon wafer is mounted in a supporting recess of a susceptor attached to a conveyor of a coating apparatus, and a layer of a material such as silicon is deposited on the surface of the wafer, for example, by heating the wafer in a reactor or a reaction chamber of the coating apparatus and supplying vapor including silicon to the reactor or the reaction chamber.
The susceptor used conventionally for forming the coating film by the in-line or continuous CVD process has a laminated structure, for example, comprising a substrate made of a heat resistant metal alloy of nickel and chromium (Inconel.TM., from International Nickel Co., Inc.) and a coating layer such as CR.sub.2 O.sub.3 coated on the surface of the substrate.
Since a susceptor of this kind is composed of a material having a high coefficient of thermal expansion and heating of the susceptor results in a heat distortion, there is a danger that a coating film produced by the CVD process may not have a uniform thickness. The thickness of the film deposited by the CVD process may be subject to an error up to about 8%.
For example, in a dual layer structure consisting of a Cr.sub.2 O.sub.3 layer on an Inconel substrate, since the heat conductivity of the Cr.sub.2 O.sub.3 layer is much smaller than that of the Inconel substrate, heat conduction through the Cr.sub.2 O.sub.3 layer is not sufficient. Accordingly, when a susceptor having such a dual layer structure is heated from below, there is a danger that the temperature at the surface of the silicon wafer may vary slightly depending on the positions on the surface thereof and elapse of time.
In addition, it is difficult to control the thickness of the Cr.sub.2 O.sub.3 layer, particularly, to control the thickness of the layer so as to be uniform for each of the susceptors. Accordingly, there is a danger that the temperature at the surface of the wafer may vary on every susceptor. Therefore, there is a possibility that an error or a difference in the thickness of the film produced by the CVD process on the silicon wafer may occur depending on location on a wafer, or on the positions of an individual wafer or wafers on each susceptor.
Further, such a dual structure may possibly cause the Cr.sub.2 O.sub.3 layer to delaminate due to the difference in the coefficients of thermal expansion between the Inconel substrate and the Cr.sub.2 O.sub.3 layer, as well as heat distortion may be caused in the susceptor, the amount of distortion increasing as the size of the susceptor is enlarged. Accordingly, heat contact with a wafer mounted in the recess of the susceptor becomes incomplete. Thus, the temperature at the surface of the wafer and the orientation of the wafer may vary on every wafer and result in variations in the manner of contact between the gas stream and the wafer or in the heat dissipation from the surface of the wafer by the gas stream on every wafer. This causes an error or a difference in the thickness and in the uniformity of the thickness of the film formed on the surface of the wafer by the CVD process.
A susceptor used for the in-line CVD process is generally produced by using an Inconel plate, for example, of about 1 mm to 2 mm thickness as a substrate and forming a supporting recess of a size required for supportingly receiving the lower portion of a silicon wafer on the surface of the susceptor, for example, by pressing. As described above, a dimensional change or difference results due to the heat distortion of the recess for supporting the silicon wafer, and the change becomes greater as the size of the susceptor is increased. Accordingly, it is not always easy to produce a susceptor having a supporting recess with a large diameter for use in coating a silicon wafer by the CVD process, particularly, a recess having a diameter of greater than five inches.
Further, it is desired that a susceptor for use in inline coating of a silicon wafer by the CVD process have enhanced heat resistance and spalling resistance, as well as has a high mechanical strength. Furthermore, it is preferred that the material of the susceptor does not generate gases. Accordingly, an intrusion of undesired impurities, however small, into the in-line or continuous coating apparatus should preferably be avoided. Cleanliness and control of the impurities are extremely important in order for a semiconductor element prepared from the wafer to have the desired basic electrical properties.